In Nairobi's Karura forest, a small group of people -- many of them survivors or bereaved families -- held inter-faith prayers at a site where a memorial stone and a plaque bearing the names of the dead, as well as newly planted trees, was unveiled a year ago.

The attack began on September 21, 2013, when four gunmen entered the mall, spraying shoppers and staff with machine gun fire and tossing grenades into crowds of Saturday shoppers and diners. The attack left 67 dead.

Apparently inspired by the Mumbai attack of 2008, the gunmen hunted down shoppers in supermarket aisles and singled out non-Muslims for execution. They then fought it out with Kenyan security forces before the siege was finally declared over four days after the first shot was fired.

"We cannot even understand how it happened, and we honestly hope that it will never happen again," said Rupal Mital Shah, whose husband was killed in the attack.

"We miss him dearly, we still love him and we are just here because we planted all these trees for all the victims."

"It was absolutely horrendous," said attack survivor Amanda Belcher. "I think everyone who was there will be scarred in their memories for the rest of their lives."

The Shebab said the attack was revenge for Kenya's sending of troops to fight the extremists in Somalia as part of an African Union force. They have launched a string of subsequent attacks in Kenya, including their biggest attack to date earlier this year -- the massacre of 148 people, most of them students, at Garissa university in the northeast.

All four gunmen were believed to have died in the mall, their bodies burned and crushed by tonnes of rubble after a section of the complex collapsed following a fierce blaze started by the fighting.

The Westgate mall, Nairobi's most upmarket shopping centre and a magnet for the east African nation's growing middle class and expatriates, reopened in July after extensive renovations. A prayer service is also expected to take place inside the mall on Monday.